1. Field of the Invention
The present invention relates to a segmented, flexible drill pipe having an improved configuration for the interlocking segments.
2. Prior Art
In drilling deviated well bores it is necessary to use drill pipe that can bend through the curve of deviation from the vertical. There have been a number of proposals for segmenting regular drill pipe to make the pipe bend without permanently distorting the pipe. For example, U.S. Pat. No. 2,515,366, issued to John A. Zublin, teaches forming a plurality of generally circumferential slots, each cut through the wall of a tubular drill pipe, so that the pipe is divided into a plurality of sections.
While the slots are generally circumferential, they deviate from a true circumference to form a plurality of interengaging and loosely interlocking teeth having a dovetail configuration. The tops and sides of each dovetail configuration, in the Zublin patent, are each relatively straight. Further, Zublin's dovetail cuts are vertically true, each being straight in line with the next.
It has been found that the straight sides of the Zublin dovetail cut allow excessive distortion in bending the drill pipe and often cause damage to the cut if excessive force is applied at the bend. Sometimes, on heavy-duty rotation of the drill pipe, the pipe will fail at a particular stress-weakened cut. This has been attributed to the vertical alignment, as well as the straight sides of the cuts.
In U.S. Pat. No. 4,328,839, there is shown a modified flexible drill pipe in which the sides of the interlocking teeth are formed in reversed intersecting semicircles to provide an improved force bearing action between adjacent teeth upon bending of the flexible drill pipe. However, it has been found that this configuration, while providing improved characteristics over the Zublin type drill pipe, nevertheless is subject to excessive elongation, and potential failure, upon substantial axial loading because of the absence of a positive stop between adjacent segments to prevent excessive axial distortion. Further, this configuration remains subject to potential breakage upon excessive bending because of the relatively narrow neck portion at the base of each of the interlocking teeth, the configuration being such that the tooth is narrowest, and therefore weakest, at the point where it is subject to the greatest concentration of bending moment forces.
Various other configurations of segmented drill pipes are shown in U.S. Pat. Nos.:
2,382,933 to Zublin; PA1 2,515,366 to Zublin; PA1 2,585,207 to Zublin; PA1 2,687,282 to Sanders; PA1 2,694,549 to James; PA1 3,160,218 to Crake; PA1 3,903,974 to Cullen; and PA1 4,067,404 to Crase.
Therefore, it is an object of the present invention to provide an improved flexible drill pipe by providing a unique configuration for interlocking teeth thereof.
It is a further object to provide increased rotational strength and tension strength in a flexible drill pipe.
Another object is to provide improved bending of the flexible drill pipe while retaining integrity of the interlocking teeth.
Another object is to provide a positive lock against excessive axial displacement between adjacent pipe segments of the flexible drill pipe.
Yet another object is to provide improved rotational strength of the drill pipe by providing an offset to each succeeding cut forming the interlocking teeth.
Another object is to provide such a flexible drill pipe with improved bearing characteristics between adjacent segments to reduce wear and improve the life of the drill pipe.